Wire and cable insulation or coating compositions are normally quite flammable. As a result, they can pose a fire hazard in power plants, distribution areas, manholes, and buildings. Ignition can easily occur from overheating or arcing. Therefore, flame retardants are generally used in wire and cable coatings to prevent electric sparks and subsequently to prevent the spread of fire along the cable.
Flame retardants such as halogenated additives (compounds based on fluorine, chlorine or bromine) or halogenated polymers, such as chlorosulfonated polyethylene, neoprene, polyvinyl chloride, or the like, are commonly used as flame retardants in wire and cable insulation or coating compositions. Both halogenated additives and halogenated polymers are capable of giving fire-resistant properties to the polymer that forms the coating. Halogens, however, have a major drawback in that the gases evolved (i.e. hydrogen chloride, hydrogen fluoride and hydrogen bromide) during burning, or even merely overheating, are corrosive as well as being toxic. As a result, the use of halogenated additives or halogenated compounds are not recommended in enclosed locations.
A second alternative for providing flame retardancy for wire and cable insulation, especially those made of polyolefins, is to use an inorganic oxide, such as aluminum, magnesium, titanium, and bismuth oxides, in hydrated form, as a filler in the polymer matrix. The hydrated metal oxide provides flame retardancy by a mechanism known as water of hydration. When the hydrated metal is heated, water is evolved which effects a flame retardant action. A drawback of this system is that the hydrated metal oxide is polar which absorbs moisture when the cable is exposed to a wet environment, resulting in a reduction in the electrical insulation properties of the coating composition.
U.S. Pat. No. 6,803,517 to Caimi discloses the addition of a silane-based coupling compound to the coating mixture to improve the compatibility of the hydrated metal oxide and the olefinic base polymer.
WO 95/10565 discloses a flame retardant cable and wire insulation comprising polyolefin, melamine as a flame retardant, polyphenylene oxide and silicaceous filling.
U.S. Pat. No. 6,025,422 to Hall discloses a flame retardant polymer composition comprising a polymer blend of aliphatic polyketone and hydrated inorganic filler.
U.S. Pat. No. 6,143,816 to Prescher et al. discloses a fire retardant plastic material comprising a plastic mixture which contains 55 to 75 percent by weight of boehmite.
WO 99/05688 discloses a low-smoke and flame-retardant cable coating comprising a polymer mixture and magnesium hydroxide as the flame retardant.
U.S. Pat. No. 5,955,525 to Sanchez et al. discloses a fire resistant, low-smoke wire insulation comprising a hydrated inorganic charge, an alkoxysilane, an additive, a hindered phenol, and a zinc-mercapto toluimidazol salt.
U.S. Pat. No. 4,001,128 to Penneck discloses a filler system for polymers which provides a high voltage insulation material which is resistant to tracking. Penneck discloses a combination of alumina trihydrate together with a chemically treated silica filler in combination with various polymeric materials as having improved tracking resistance in high voltage insulating applications. Ethylene-vinyl acetate copolymers are disclosed among those which may be modified utilizing the disclosed fillers. Such insulating materials may also be crosslinked.
U.S. Pat. No. 4,022,945 to MacKenzie, Jr., et al discloses a fire retardant system for polymeric compositions comprising a combination of a halogen-containing hydrocarbon and an oxide of iron. The disclosed compositions may include ethylene-vinyl acetate copolymers in admixture with various modifiers in addition to the specific flame-retardant agents disclosed as components of the insulation material.
None of the documents discussed above address the problem of moisture resistance in a fire resistant cable insulation. As has been previously discussed, exposure of a fire resistant cable to a wet environment significantly reduces the dielectric insulation properties of the cable coating, which can be a significant problem for underground cables, cables used on ships, and under water cables.
WO 98/40895 discloses a fire-resistant, moisture resistant cable coating comprising two layers of coating. An inner layer constructed to impart moisture resistance to the coating; and an outer layer constructed to impart fire-resistance to the cable.
U.S. Pat. No. 5,912,436 to Sanchez et al. discloses a cable coating comprising three layers. A layer designed to impart moisture resistance; a layer designed to impart low-smoke emission; and a layer designed to impart flame-retardant properties.
Although both WO 98/40895 and U.S. Pat. No. 5,912,436 address low smoke emission, fire-resistance and moisture resistance in a coating, the coating comprises multiple layers which significantly increases the construction cost and complexity of the cable.
Applicant has developed a new cable coating capable of low smoke emission, fire-resistance and moisture resistance in a single layer. The new cable coating has improved wet electrical performance and satisfactorily meets small gauge (#14 AWG) fire performance VW-1 rating, and limited smoke characteristic as defined in UL 1685 for cable tray usage.